1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for fabricating the same. More particularly, the present invention relates to a liquid crystal display device having excellent display quality, impact resistance and pressure resistance, as well as a liquid crystal display device which can be obtained at a lower cost with excellent productivity, and a method for fabricating the same.
2. Description of the Related Art
Liquid crystal display devices having various display modes have been known in the art, including a liquid crystal display device having a non-scattering mode such as a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringency (ECB) mode, a ferroelectric liquid crystal (FLC) mode; and a liquid crystal display device having a scattering mode such as a polymer dispersed-type liquid crystal (PDLC) mode.
A conventional liquid crystal display device includes a pair of substrates and a display medium containing at least liquid crystal interposed therebetween. When such a liquid crystal display device is utilized for a portable information terminal having a pen-input function, there is a problem that the orientation of the liquid crystal molecules will be disturbed due to the reduction of the gap between the substrates caused by the pressure at the time of the pen input, which results in the disturbance of the display in the liquid crystal display device.
In order to solve such a problem, a liquid crystal display device having improved impact resistance and pressure resistance has been proposed by providing a polymer wall or polymer columns in the periphery of the liquid crystal region to reduce the variation of the gap.
For example, a liquid crystal display device comprising a polymer column selectively provided in a non-pixel portion using a resist and which is used as a spacer (Japanese Laid-open Patent Publication No. 56-99384), and a liquid crystal display device comprising a polymer column selectively provided in stripes in a non-pixel portion using a photosensitive resin (Japanese Laid-open Patent Publication No. 59-201021), have been known.
In Japanese Laid-open Patent Publication No. 6-301015, the applicant proposed a liquid crystal display device shown in FIG. 3A as having improved impact resistance. The liquid crystal display device 300 includes a pair of substrates 101a and 101b, and a display medium interposed therebetween, the display medium having a liquid crystal region 106 provided in a pixel portion and a polymer wall 107 surrounding the liquid crystal region 106. In the fabrication of the liquid crystal display device 300, as shown in FIG. 3B, a mixed material 113 containing a liquid crystal material, a photopolymerizable monomer and a photopolymerization initiator is injected between a pair of substrates 101a and 101b, and a photomask 114 having a light-shading portion 110 and a light-transmitting portion 111 is disposed on one of the substrate 101a so that the pixel portion is covered with the shading portion 110, and then the mixture 113 is irradiated with ultraviolet light 108 through the photomask 114. The photopolymerizable monomer is polymerized and the resulting polymer gathers in the portion corresponding to the light-transmitting portion 111 (irradiated portion) of the photomask 114, while the liquid crystal material gathers in the portion corresponding to the light-shading portion 110 (non-irradiated portion), which results in a display medium having the liquid crystal region 106 in the pixel portion and the polymer wall 107 in the non-pixel portion. According to this liquid crystal display device, it is possible to improve the impact resistance by using the polymer wall 107 provided in the non-pixel portion, as shown in FIG. 3A. It is also possible to accomplish wide viewing angle characteristics in the liquid crystal display panel by axisymmetrically orienting the liquid crystal molecules in the pixel due to the interaction between the polymer and the liquid crystal material. Such a mode as axisymmetrically orienting the liquid crystal molecules in the pixel is hereinafter referred to as an axisymmetrically orienting mode.
Moreover, the applicant proposed in Japanese Laid-Open Patent Publication No. 7-287241 a liquid crystal display device which provides a liquid crystal region in the pixel portion without any photomask by using a transparent electrode which does not readily transmit light, and also enables the polymer wall to precisely conform to the outlines of the pixel and the liquid crystal region.
However, the liquid crystal display device having such a polymer wall or a polymer column as discussed above has the following problems.
The liquid crystal display device having a polymer wall or a polymer column formed from a resist or a photosensitive resin requires an additional photolithography step so as to precisely form the polymer wall or the polymer column in the non-pixel portion. Therefore, such a liquid crystal display device provides very poor production efficiency.
In such a liquid crystal display device as shown in FIG. 3A, it is necessary to dispose the photomask 114 so that the light shading portion 110 of the photomask 114 conforms to the pixel portion. Therefore, again, such a liquid crystal display device provides very poor production efficiency.
A liquid crystal display device using a transparent electrode which does not readily transmit a light having the prescribed wavelength provides improved productivity, since it is not necessary to adjust the location of the photomask, and the like. However, the polymer wall may be undesirably formed over the pixel portion, or polymer pieces may remain in the pixel portion in such a liquid crystal display device, which results in a reduction of the aperture ratio of the liquid crystal display device which causes a darkening of the display, and the poor orientation of the liquid crystal molecules causes increased roughness of the display. Moreover, sometimes the liquid crystal region is not sufficiently surrounded by the polymer wall in the liquid crystal display device, and in this case the display device has insufficient pressure resistance.
In such a liquid crystal display device, a method of precisely providing the liquid crystal region in the pixel portion and the polymer wall in the non-pixel portion by suitably phase-separating the liquid crystal material from the polymer has been employed. Thus, the polymer wall is prevented from being undesirably formed over the pixel portion and thus from affecting the display quality. Additionally, the liquid crystal region is surrounded by the polymer wall, thereby improving the impact resistance of the liquid crystal display device. However, in this method, since a special thin layer or a special process for improving the degree of separation of the liquid crystal material from the polymer is required, the production cost is increased.
As described above, a liquid crystal display device having excellent display quality, impact resistance and pressure resistance, which can be obtained at a lower cost with excellent productivity, has been demanded.